PROJECT SUMMARY/ABSTRACT Rotaviruses (RVs) are highly infectious viruses of great importance because they are the most common cause of severe gastroenteritis in young children. We will address three fundamental topics in rotavirology (RV) that will expand our understanding of the molecular mechanisms regulating RV innate immune evasion. 1. Determine the structural basis and in vivo activity of NSP1-mediated ?-TrCP degradation. Despite the RV NSP1 protein's well-documented ability to induce IRF3 and/or ?-TrCP degradation, the mechanisms regulating this degradation are unknown. Human RV NSP1s specifically target ?-TrCP. We have recently identified an unexpected role of the host Cullin-E3 ligase complex in NSP1's degradative functions. In this aim we will dissect how NSP1 is able to hijack the host Cullin-E3 ligase complex, induce ?- TrCP degradation, block NF-?B activation and thereby promote homologous RV replication. 2. Identify the molecular mechanisms underlying MAVS degradation by VP3 in a strain- and host- specific fashion both in vitro and in vivo. We previously showed that ssRNA byproducts from RV infection are potent activators of cytosolic sensors RIG-I and MDA5, both of which converge on mitochondrial antiviral signaling protein (MAVS) to relay innate signaling and induce IFN expression. Unexpected preliminary findings suggest that MAVS is targeted for proteasomal degradation by the RV VP3 protein in a host range restricted (HRR) manner. Here we will explore the complex interplay between VP3 and MAVS from different RV species at a mechanistic level and evaluate the importance of VP3-mediated MAVS degradation in promoting RV replication in vitro and in vivo. 3. Identify the mechanism of RV NSP1-mediated inhibition of STAT1 activation and the intestinal cell origin of the IFN responses to RV infection. Despite the ability to efficiently suppress the induction of type I IFN in intestinal epithelial cells (IECs), homologous RV infection still induces substantial levels of type I and III IFNs in the gut. This IFN production suggests that RVs must be able to subvert IFN-mediated antiviral amplification as well as blocking IFN induction. RV NSP1 efficiently inhibits IFN-mediated STAT1 phosphorylation. New findings indicate RV blocks STAT1 activation by depleting multiple IFN receptors, likely by NSP1-directed degradation. RVs can also block IFN-directed STAT1 activation in uninfected cells in vitro. Whether this effect also occurs in vivo is unknown. We propose to identify the hematopoietic cell and IEC origins of type I and III IFNs elicited by RV infection. We will also examine the mechanistic determinants of RV-mediated IFN receptor degradation and inhibition of STAT1 activation and determine if differences in these functions contribute to RV HRR.